1. Field of Invention
The present invention relates to an apparatus for correcting fluctuations in current through a semiconductor circuit so that its dissipation power can be reduced and, more specifically, technologies of a circuit current generation apparatus and method thereof, and a signal processing apparatus.
2. Description of Related Art
Recently, portable devices represented by portable telephones have come to be mounted with a variety of receivers, so that it has been required to greatly reduce power dissipated by the receivers. For example, some receivers used in analog TV sets conventionally dissipate power in excess of several hundreds milliwatt to one watt and so are not qualified for use in portable devices driven by batteries. However, digital TV sets for portable use to appear aftertime are required for a great cutback in dissipation power than ever; receivers thereof are required to have dissipation power of 100 mW or less.
To meet these requirements, circuits that constitute the receiver are mostly incorporated into a semiconductor integrated circuit, so that dissipation power of the semiconductor integrated circuit (hereinafter abbreviated as IC) determines that of the receiver.
However, manufacturing processes for an IC fluctuate significantly to fluctuate dissipation power of the IC by at least ±20% in some cases, so that a ratio between a maximum value and a minimum value of its current may become at least 1.5. Circuit performance of the receiver depends largely on high-frequency circuits. Further, a current value of the high-frequency circuits is larger than that of other bias circuits and low-frequency circuits; each of transistors thereof may require several milliamperes or more in some cases. Therefore, by reducing a current of the high-frequency circuit, dissipation power can be reduced effectively. Moreover, characteristics of the high-frequency circuit depend on a value of current through the circuit in many cases; in particular, an oscillation circuit in a local oscillator etc. may encounter a fatal problem of disruption of oscillation caused by a reduction in current. Therefore, how much a current can be reduced is limited by a value of a lowest current due to current fluctuations, thus constituting one of problems to be solved in reduction of dissipation power. Therefore, suppressing fluctuations in current is one of the important conditions for lower dissipation power.
Conventionally, a current value of a high-frequency circuit has been set with a sufficient margin for the purpose of prioritizing characteristics of the circuit, so that if the current value was too high, dissipation power of the circuit has been reduced to prevent a rise in temperature. For example, Japanese Patent Application Laid Open Publication No. Hei 9-107299 discloses taking countermeasures against a rise in dissipation current switchover temperature through a reception electric field. However, currently no measure is taken to solve deterioration in characteristics caused by fluctuations in current owing to fluctuations in manufacturing of an IC.
To solve these problems, a conventional method reduces a current of a high-frequency circuit itself and so has a problem of a deterioration in characteristics at a lowest current owing to current fluctuations as described above, which current fluctuations constitute an obstacle against a reduction in dissipation power of an IC because of a necessity to secure a margin for the current fluctuations.
Further, by reducing fluctuations in the processes and screening components in manufacturing processes, current fluctuations can be reduced, which approach, however, has a problem of increased costs and deteriorated productivity.